1. Field of the Invention
The present invention relates to a method of converting data of a first colorimetric system into data of a second colorimetric system, which include data outside a color reproduction range, and thereafter mapping the converted data into the color reproduction range.
2. Description of the Related Art
Color reproduction ranges, i.e., ranges in which colors can be reproduced, vary depending on the medium used and the output device used. For example, a range of the colors of a color image displayed on a CRT display differs from a range of the colors of a color image outputted from an image output device such as a color printer or the like. When color data supplied from a first color data processing device is supplied to a second color data processing device which has different characteristics from the first color data processing device, there may be colors from the first color data processing device that cannot be processed by the second color data processing device. If there are such colors, then it is customary to carry out a gamut mapping process for mapping color data outside a color reproduction range of the second color data processing device into the color reproduction range.
According to a known process disclosed in Japanese laid-open patent publication No. 4-196675, for example, color data of a first color data processing device are converted into color data in a common color space such as an L*u*v* colorimetric system independent of the first color data processing device, then L*, u*, v* values of the color data in the common color space are compressed on the basis of a desired relationship, and thereafter the compressed data are converted into color data in a color space inherent in a second color data processing device.
If the color data are mapped in the common color space according to a gamut mapping process, then the mapped color data may not be desirable data for use in the second color data processing device, and a color image reproduced on the basis of the color data thus obtained may look unnatural as compared with the original color image. For this reason, the color data should be mapped in the color space inherent in the second color data processing device.
If the inherent color space comprises color data of four variables C (cyan), M (magenta), Y (yellow), K (black) (which may hereinafter be referred to as color data CMYK), e.g., color data for use in color printing, and also if the four-variable color data CMYK in the inherent color space which correspond to three-variable color data in the common color space are outside a color reproduction range, then the color data CMYK need to be mapped into the color reproduction range. It is noted that the variable K may be represented as a function of one or more of the variables Y, M, C. However, the K variable is often described as a fourth component in the YMCK color space. In the mapping process, depending on settings for the color data K, colors may change greatly or may become different in the vicinity of the color data mapped from outside the color reproduction range. Stated otherwise, for converting color data of three variables into color data of four variables, it is necessary to determine a conversion relationship by fixing one of the variables to a given value. If such one variable is color data K, then settings for the color data K greatly affect the mapping process.